Shinde M N, Khurana R, Barooah N, Bhasikuttan A C, Mohanty J
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
Org Biomol Chem. 2017 May 10;15(18):3975-3984. doi: 10.1039/c7ob00506g.
The host-guest interactions and the consequent modulation in the prototropic equilibrium of a phenazine dye, neutral red, with p-sulfonatocalix[4]arene (SCX4) and p-sulfonatocalix[6]arene (SCX6) macrocyclic hosts have been investigated. Both the neutral (NR) and cationic (NRH) forms of dyes formed inclusion complexes with SCX6, with a larger binding constant for the latter (K = 8.6 × 10 Mversus 4.8 × 10 M) due to the cation receptor behavior of the calixarenes. The distinct differences in the binding constant of NR and NRH provided a finite tuning of pK between 6.5 and 8.8, through a competitive binding with metal ions. Importantly, the fluorescence quenching observed in the SCX-neutral red interactions stands in contrast to the fluorescence enhancement observed with other macrocyclic hosts, such as β-cyclodextrin and cucurbit[7]uril. This is due to the unique portal stacking interaction of NRH with the SCXs, compared to the axial inclusion geometry documented for the other macrocycles. The electron transfer from the SCX to the neutral red dye is adjudged to be the effective excited-state relaxation pathway leading to fluorescence quenching. In combination with the metal-ion induced fluorescence regeneration and tuning the pK value, the SCX-neutral red system finds potential applications in drug delivery, photodynamic therapy, catalysis, and sensor applications.
研究了吩嗪染料中性红与对磺酸钠杯[4]芳烃（SCX4）和对磺酸钠杯[6]芳烃（SCX6）大环主体之间的主客体相互作用以及由此引起的质子转移平衡的调节。染料的中性（NR）和阳离子（NRH）形式均与SCX6形成包合物，由于杯芳烃的阳离子受体行为，后者的结合常数更大（K = 8.6×10⁵ M对4.8×10⁴ M）。NR和NRH结合常数的明显差异通过与金属离子的竞争性结合，在6.5至8.8之间提供了有限的pK调节。重要的是，在SCX-中性红相互作用中观察到的荧光猝灭与在其他大环主体（如β-环糊精和葫芦[7]脲）中观察到的荧光增强形成对比。这是由于与其他大环化合物记录的轴向包合几何结构相比，NRH与SCXs具有独特的门户堆积相互作用。从SCX到中性红染料的电子转移被判定为导致荧光猝灭的有效激发态弛豫途径。结合金属离子诱导的荧光再生和调节pK值，SCX-中性红体系在药物递送、光动力疗法、催化和传感器应用中具有潜在应用。
